The Hacksmith, a mechanical engineer who has made full exoskeletons, a Batman grappling device, and Batarangs, among other things, made this Captain America arm bracer and shield using vibranium a pair of powerful electromagnets (which are available on Amazon) running on a set of lithium-polymer batteries from Hobby King, giving the shield more than 3000 lbs of magnetic lifting force. In the demonstration video, the electromagnets are being run at extremely high voltage, which is only possible for a minute, but they can be run at lower voltage for a longer period of time.

Here's his making-of video, in case you wanted to make your very own star-spangled boomerang shield: